Recovery of carbon dioxide



June 25, 1940, G, T, REIC 2,205,962

RECOVERY CARBON DIOXIDE Filed July 15, 1938 2 Sheets-Sheet 2 Exit Gas 3l 2 g 25: 3: ABSO-QBTION 4: YZJWEB F/ue E l m Gas 5022/1 /22 R 11DENSITY Q gzaumroza b Az'r V917!- 9 n F/ 2 DEAEEANON Ton 2 g \1 ESTORAGE T I'd/VA is HEAT l J EXCHANGEB 79 PUMP INVENTOR.

GUSTAVE 7: REICH A TTORNE Y Patented June 25, 1940 i UNITED STATESPATENT OFFICE RECOVERY OF CARBON DIOXIDE Gustave T. Reich, Philadelphia,Pa.

Application July 15,1938, Serial No. 219,444

6 Claims. (01. 23-150) This invention relates to the recovery of carbonducing a greater proportion of bicarbonate in dioxide and isparticularly directed to an imthe portion of the slurry that isthickened. The proved method of recovering carbon dioxide from thickenedportion of the slurry is .then heated gases containing this substance byabsorbing the and conveyed to the thermal decomposer. The carbon dioxidein alkali carbonate solutions and heating of the thickened slurryprevious to de- 5 decomposing the alkali bicarbonate thus formedcomposition may be accomplished very convenby heat, iently by passing itin contact with the hot car- A principal object of the invention is theprobon dioxide from the decomposer. This carbon vision of a methodwhereby the recovery of cardioxide gas carries with it much moisturewhich I 10 Icon dioxide may be effected more efficiently, with isremoved d absorbed into the shirlyin p 10 lower heat expenditure andsmaller space requireing through it and therefore passes back into m ntsthan her tofor the decomposer. A further heating of the slurry,

-In the processes for the recovery of carbon dibefore a h h d p s ay bet d oxide, as practiced heretofore, the gases cony Passing it inCounter-current through a e taining the carbon dioxide have beencontacted exchanger with the carbon dioxide leaving th 16 with alkalicarbonates or such concentration and deeomposer- In the decomposer. cion of for such time that at the completion of. the abthe bicarbonate insolution and suspended bicarsorption operation the alkali bicarbonate iswholly bonate takes place. The continuous feeding of in solution. It hasnow been found that the the slurry into the decompose! Continuallybrings 20 efliciency of the recovery process may be very into it freshportions of suspended bicarbonate, m greatly increased by carrying outthe absorption which in turn is absorbed into solution and deoperationunder such conditions that a substancomposede liquid from which thecarbon tial amount of alkali bicarbonate is produced in dioxide has beenliberated and which is large y the solid form, thus forming a suspensionof solid 3 solution of lk Carbonate, though i m y alkali bicarbonate ina saturated solution of alcontain greater or less amounts of alkali,bicar- 25 kali bicarbonate containing greater or lesser bonate, is thenconveyed back to the absorption amounts of alkali carbonate in solution,and by w The carbon dioxide gas isv t k n t a decomposing at least aportion of the alkali bisas r or directly into us e n r qcarbonatesuspension thus formed by heating the Hid from the thickener y bereturned c y suspension, whereby the alkali bicarbonate is to theearbonating tower. It will contain chiefly 3 decomposed int carbo di idewhi h i alkali carbonate and alkali bicarbonate in solumoved in gaseousform and alkali carbonate tiOn, and p ps a small amount of alkalibiwhich remains in solution and is returnedto the carbonate insuspension absorption operation. Further increase in efli- The inventionwill be more particularly. de-

ciency may be obtained by thickening the susscribed, for the p p ofillustrating the p 35 pension of alkali bicarbonate, as by allowing itoiple involved. with reference t the a mpanyto settle in the absorptionchamber or elsewhere in dr wi g in wh h! or in other suitable manner,before subjecting it 1 is a diagrammatic View of one embodito thedecomposing operation. I ment of the invention, including a thickener;and The absorption may be carried out either unde Fig. 2 i a a r mmatiview of another em- 40 I atmospheric or super-atmospheric conditions. Itbodiment of the invention in which the thickener is particularlydesirable that the concentration is'not used. of alkali carbonate andbicarbonate be main- A sodium carbonate solution at a temperature tainedbelow the point at which the double salt of 30 to 50 C. and containing12 to 18 pounds'of of alkali carbonate and alkali bicarbonate is sodiumcarbonate and 5-to 12 pounds of bicar- 45 formed in order to avoiddifliculties in handling bonate per cubic foot in solution is pumpedinto and decomposing the suspensions of alkali bicarthe carbonationtower I. This tower, in conbonate. For the sodium carbonate-bicarbonatestruction and operation, may be similar to the system concentration ofthe solutions should prefusual soda ash carbonation tower or it may be5o'erably' be less than 100 grams of soduim (Na) per in the formdescribed in my application Serial liter, at a temperature of 25 C. No.116,041, filed December 15, 1936, and may be The thickening of theslurry may be effected preferably operated under pressure, depending ina Dorr thickener or other suitable device that upon the concentration ofthe flue, gas. The permits the thinnerpart of the liquid'to be drawnmore dilute the flue gas the higher the pressure off, thickening therest of the slurry and proused, so that an economical carbon dioxide rera oovery is possible. After carbonation the mixture not only containssodium bicarbonate in saturated solution, but also sodium bicarbonate insuspension. This suspension may be agitated to prevent the deposit ofthe solid bicarbonate. By means of a density thickener, a liquid may becontinuously withdrawn which may contain in solution and in suspension atotal of 15 to 30 pounds of sodium bicarbonate per cubic foot. Suchdensity may also be obtained by withdrawing the carbonated liquidcontinuously into a Dorr thickener, or similar mechanical device, andthe excess sodium bicarbonate contained in the carbonate solutionsreturned to the carbonation tower.

The mixture containing sodium bicarbonate in solution and suspension maythen pass into the deaerating tower 3. This tower need only be usedshould the concentrated sodium bicarbonate solution contain some air,which must be removed so that the carbon dioxide recovered afterwardshall have a purity of at least 99.5% carbon dioxide. The deaeratingtower may be either a tower provided with bafile plates, as shown in thedrawings, or any suitable equipment which will not cause separation ofthe suspended sodium bicarbonate from the dissolved sodium bicarbonate.

The deaerated mixture may then pass into the preheater through which thehot carbon dioxide gas, saturated with water vapor, passes. Thepreheater 4 serves first for the preheating of the sodium bicarbonatesolution by direct contact with the hot carbon dioxide gas coming fromthe decomposer. After passing through it, the carbon dioxide gas iscooled before it is collected, preferably in the gasometer. Anotherfunction of this preheater is to reduce the water carried away with thehot carbon dioxide gas. As the concentration of the alkali solutionpassing to the carbonation tower should be constant to obtain the bestresults, the water carried away with the hot carbon dioxide must bereplaced. However, by returning same with the alkali solution passingthrough the preheater a constant concentration can readily bemaintained, and the sensible heat in the vapors helps to preheat theincoming alkali bicarbonate slurry. The preheater may be of the sameconstruction as the deaerator or be any suitable equipment which willnot disturb the equilibrium between the suspended and dissolved sodiumbicarbonate.

From the preheater the alkali'solution passes through a heatexchanger 1. Here the incoming sodium bicarbonate suspension ispreheated by means of the sodium carbonate solution coming from thedecomposer. The sodium carbonate solution coming from the decomposer ispreferably not cooled below 70 C. (or not below the temperature at whichsolids precipitate out) before going to the carbonation tower or beforebeing mixed with liquor free from suspended solids. The heat exchangeron the incoming sodium bicarbonate liquid side is vented to thepreheater, so that any carbon dioxide given off in the heat exchangermay pass to the gasometer also.

The heat exchanger is preferably provided with an agitator so that thesolids will be kept in suspension and not settled out. A rotary tubularheat exchanger may also be used or any mechanical means preventing theseparation of solids from the liquid whereby a good conductivity of heatis effected between the incoming sodium carbonate and the outflowing hotsodium carbonate solution.

The preheated sodium bicarbonate solution passes now from the heatexchanger into the decomposer 9. This is preferably a continuousapparatus such as a tubular boiler that revolves so that no incrustationor sedimentation of the suspended sodium bicarbonate will occur. It maybe a shell evaporator provided with bafile plates or agitators or both,so that a slow, gradual decomposition, and no mixing of progressivelydecomposed solution takes place. The heating may either be by steam oropen fire, depending upon the local conditions. The carbon dioxide whichis given off by the decomposition of the sodium bicarbonate passes firstthrough the preheater for the purpose stated above. After being freed ofits excess water and cooled to the proper temperature, it is passed intothe gasometer from whence it is pumped to the compressor, purified andliquefied. The hot, largely decomposed bicarbonate solution, afterpassing through the heat exchanger and being cooled somewhat, isreturned to the carbonation tower and is recarbonated. This cycle ofoperation is repeated thus giving the process a continuity of operation.

The processes above described may be continug."

ous, no foreign chemicals are employed, therefore no contamination cantake place.

The decomposing temperature is preferably above C., for example, to C.so that It is a quick decomposition may take place. sometimes advisableto use pressure, which is a common procedure in the carbon dioxiderecovery plants. If pressure is used, provision must be made to feed thesuspended material in at an even rate.

The general flow of the alkali solution in the mode of operationdescribed above by way of example is as follows:

The solution flows through the carbonation tower wherein alkalicarbonate is converted to alkali bicarbonate with the formation of asuspension or slurry containing suspended bicarbonate. The suspensionfrom the carbonation tower may be separated into a thickened portion anda liquid portion containing substantially no suspended bicarbonate whichremains in or is returned to the tower. The suspension or the thickenedportion of the suspension is passed through the decomposer whereinalkali bicarbonate is dissociated to alkali carbonate and carbondioxide. The solution of alkali carbonate, mixed, if desired, with theliquid portion separated from the suspension, is returned to thecarbonation tower, diluting if necessary.

It will be seen that the invention is subject to a large number ofVariations without departing from the characteristic principle that thecarbonation of the alkali carbonate solution is continued until asuspension of alkali bicarbonate is formed therein and that thesuspension or slurry of alkali bicarbonate, either as formed or alterthickening, is subjected to heating to thermally decompose at least aportion of the alkali bicarbonate into carbon dioxide and alkalicarbonate, the formation of the unmanageable double salt of alkalicarbonate and bicarbonate being avoided by keeping the totalconcentration below that at which the double salt will form under theconditions of operation. The process can, of course, be operated withpotassium carbonate and bicarbonate or with a mixture of sodium andpotassium carbonates and *bicarbonates.

This application is a continuation-in-part of my application Serial No.697,966, filed November 14, 1933.

Iclaim:

l. A process of isolating carbon dioxide from line gases which comprisesabsorbing the carbon dioxide in an alkali carbonate solution until thereis produced a slurry containing alkali bicarbonate in suspension,thickening the slurry and thermally decomposing the thickened slurry ata tornperature above C. whereby carbon dioxide is evolved and theformation of the double salt of alkali carbonate and bicarbonate issubstantially prevented.

2. A process as defined in claim 1 wherein the hot misture-containingcarbon dioxide gas evolved in the decomposition is passed through thethickened slurry prior to its decomposition whereby the slurry is heatedand diluted.

3. A process as defined in claim 1 wherein the alkali bicarbonate slurryis deaerated before it is thermally decomposed.

4. A process of isolating carbon dioxide from flue gases which comprisesabsorbing the carbon dioxide in an alkali carbonate solution until thereis produced a slurry containing alkali bicarbonate in suspension,thickening the slurry to a concentration of about 15 to 30 pounds ofalkali bicar bonate per cubic foot of liquid and thermally decomposingthe thickened slurry at a temperature above 50 C. whereby carbon dioxideis evolved and the formation of the double salt of alkali carbonate andbicarbonate is substantially prevented 5. A process of isolating carbondioxide from flue gases which comprises absorbing the carbon dioxide inan alkali carbonate solution until there is produced a slurry containingalkali bicarbonate in suspension, thickening the slurry by removingtherefrom a portion of the liquid phase substantially free fromsuspended alkali bicarbonate and thermally decomposing the thickenedslurry at a temperature above 50 C. whereby carbon dioxide is evolvedand the formation of the double salt of alkali carbonate and bicarbonateis substantially prevented.

6. A process for the recovery of carbon dioxide from flue gases whichcomprises absorbing the carbon dioxide in an alkali carbonate solutionuntil at a temperature at least as high as 30 C. until a suspension ofalkali bicarbonate containing a substantial proportion of solid alkalibicarbonate is formed therein while maintaining the concentration ofalkali carbonate and bicarbonate below'the point at which the formation,of

the double salt of alkali carbonate and bicarq s'rAvE 'r. REICH.

Patent No. 2,205,962.

CERTIFICATE OF CORBECTI ON.

June .25,l 9l O. GUSTAVE T. REICH.

It is hereby certified that error appears. in theprinted specificationof the above numbered patent requiring correctionas follows: Page 5,first column; line 15, claim 2, for "misturecontaining"readmoisture-containing-; and second column, line 16, claim 6, strikeout "until" before "at"; I and that the said Letters Patent should beread with this correction therein that the same may conform to therecord of the case in the Patent Office. a A

Signed and sealed this 50th day of July, A. D. 1911.0.

Henry Van Arsdale, (Seal) Acting Commissioner of Patents.

